His other co-founder left WhatsApp in late 2017 and has been a vocal opponent of Facebook ever since.

In 2014, Facebook bought Jan Koum's messaging app and turned him into a billionaire overnight.

Now the WhatsApp co-founder is leaving his company, noting in a post on Facebook that it's "time to move on."

Though Koum didn't mention it in his public statement on Monday afternoon, The Washington Post reported that he was unhappy with Facebook's attempts to weaken WhatsApp's encryption and its desire to collect and use more WhatsApp users' personal data.

"I'm grateful for everything you'd done to help connect the world," Zuckerberg wrote, "and for everything you've taught me, including about encryption and its ability to take power from centralized systems and put it back in peoples' hands. Those values will always be at the heart of WhatsApp."

Koum's co-founder, Brian Acton, left WhatsApp in late 2017 and has been a vocal opponent of Facebook ever since. In March, Acton encouraged his Twitter followers to delete their Facebook accounts. "It is time," he said.

Koum's departure follows other reports of high-level internal disagreement at Facebook. Last month, Facebook's chief information security officer, Alex Stamos, confirmed plans to leave the company this summer. Stamos reportedly clashed with executives over the company's refusal to more publicly acknowledge the platform's role in spreading propaganda and disinformation from countries seeking to influence foreign elections.